Amalgam decomposition

ABSTRACT

A METHOD OF DECOMPOSING AN ALKALI METAL AMALGAM BY CONTACTING IT WITH WATER AND GRAPHITE BALLS HAVING A PORTION OF THEIR SURFACE COATED WITH AN UNDERLYING FERROUS METAL AND AN OVERLYING NON-FERROUS, AMALGAM-RESISTANT METAL. THE DECOMPOSITION REACTION PRODUCES HYDROGEN, AN ALKALI METAL HYDROXIDE WITH A CONCENTRATION ABOVE 50% BY WEIGHT, AND MERCURY IN WHICH THE SODIUM CONTENT IS REDUCED TO LESS THAN 0.01 PERCENT.

United States Patent Oflice 3,595,614 AMALGAM DECOMPOSITION James M. Ford, Richard W. Ralston, and Walter J.

Sakowski, Cleveland, Tenn., assignors to Olin Corporation No Drawing. Filed Nov. 21, 1969, Ser. No. 878,927 Int. Cl. B01j 11/06, 11/22; C01d 1/00 US. Cl. 23-184 15 Claims ABSTRACT OF THE DISCLOSURE A method of decomposing an alkali metal amalgam by contacting it with water and graphite balls having a portion of their surface coated with an underlying ferrous metal and an overlying non-ferrous, amalgam-resistant metal. The decomposition reaction produces hydrogen, an alkali metal hydroxide with a concentration above 50% by weight, and mercury in which the sodium content is reduced to less than 0.01 percent.

This invention relates to improvements in amalgam decomposers which are used in conjunction with mercury cathode chloralkali cells. More particularly this invention relates to improvements in the electrode material used to discharge the amalgam in such decomposers and provides means for efiicient decomposition of the amalgam.

Horizontal mercury cells usually consist of an enclosed, elongated trough which slopes slightly toward one end. The cathode is a flowing layer of mercury which is introduced at the higher end of the cell and flows along the bottom of the cell toward the lower end. The anodes are generally composedvof rectangular blocks of graphite suspended from conductive lead-ins so that the bottom of the graphite anode is spaced a short distance above the flowing mercury cathode. An aqueous electrolytic solution, for example, a brine of sodium chloride is fed to the upper end of the cell, covering the anodes and flowing concurrently with the mercury. The impressed electric current passing through the electrolytic solution between the anodes and the mercury cathode liberates chlorine at the anodes and sodium is dissolved in the mer cur as an amalgam. The sodium amalgam flows from the lower end of the cell to a decomposer where it is contacted with water to form sodium hydroxide, hydrogen and mercury. The mercury is recycled to the cell for reuse as cathode. Other aqueous electrolytes may be used particularly brines of alkali metal halides, for example, potassium chloride and lithium chloride and also sodium sulfate.

Other anodes than graphite are also well known for use in mercury cells including particularly titanium anodes at least partially coated with a thin layer of platinum metal. Included in the term, titanium are alloys consisting essentially of titanium which are also suitable for the fabrication of anodes. The term platinum metal includes an element of the group consisting of ruthenium, rhodium, palladium, osmium, iridium and platinum or alloys of two or more of these metals or oxides thereof.

The decomposition reaction between the amalgam and water does not proceed readily due to the high overvoltage of hydrogen on a mercury surface. Discharge of hydrogen gas must be facilitated by contact of the amalgam and water with a discharge electrode material, commonly graphite. The decomposer is usually packed with particles of graphite for this purpose.

Other hydroxylic liquids can be substituted for water including lower aliphatic alcohols containing one to four carbon atoms and aqueous solutions, for example, of caustic soda.

In an effort to improve the reactivity of graphite in decomposers, various modifications have been suggested.

3,595,614 Patented July 27, 1971 US. Pat. 709,971 of 1920 discloses carbon rods wound with wire or partially coated with electrolytically deposited iron. US. Pat. 739,140 of 1903 discloses carbon rods containing a small percentage of metallic iron. US. Pat. 741,864 of 1903 discloses wire wound carbon and pieces of partially-metal-coated carbon. Y

Space limitations in modern mercury cell plans do not permit the generous proportionsrequired using spaced carbon rods in amalgam decomposers or many other early designs. Modern decomposers are usually compact chambers packed with crushed graphite about A to /2',' on a side. US. Pat. 2,083,648; 2,335,045; 2,422,351 and 3,215,614 show examples of packed decomposers. These packed towers are simple and cheap to construct, main tain and operate and provide the decomposition of large volumes of amalgam. Decomposition here means removal of the alkali metal from the amalgam. I

Attempts to increase the throughput of a vertical decomposer by charging ever-increasing amounts of amalgam and water eventuall results in incomplete decomposition of the amalgam. Residual sodium in the recycle mercury leads to increased amounts of discharged hydrogen in the chlorine which may form explosive gas mixtures. Incomplete decomposition results primarily from the loss of activity of the discharge electrode material in the decomposer. Thus the problem resides in improving the activity and life of he discharge electrode material charged to the decomposer without contaminating the products due to disintegration of the discharge electrode material.

Crushed graphite, usually from broken anodes, has the advantage of low cost, since the broken anodes are otherwise discarded. However, graphite balls about 15 mm. in diameter have recently become readily available. They have the advantage of uniformly close packing which reduces the mercury inventory residing in these amalgampacked decomposers. The balls roll readily when it is necessary to remove them from the decomposer and repack it.

A disadvantage of the broken graphite or balls is their loss of activity after various periods of use, necessitating replacement or some reactivating treatment. Obviously it is advantageous for the graphite packing to retain its initial activity for as long as possible. In applying the teachings of the prior art, the graphite balls were coated with iron to cover them partially. Activity in the decomposer was greatly enhanced but was shortlived because the iron corroded and/or eroded off and the resulting caustic soda was contaminated with iron. Manganese contained in the iron used in early experiments also contaminated the caustic soda.

According to the present invention, graphite balls are washed with water, dried, and flame-sprayed first with a ferrous metal and then with nickel to apply over a portion of their surface a tightly adherent dual coating of underlying ferrous metal and an overlying coating of nonferrous, amalgam-resistant metal. 1

The graphite balls used according to this invention are suitably about 5 to 500 millimeters in diameter, although diameters of 10 to 25 millimeters are preferred. It is advantageous, due to hydrogen evolution in the decomposer, to pack smaller balls at the bottom and larger balls at the top of the decomposer. These sizes tend to remain segregated in use and they increase the hydrogen escape velocity without carry over.

Suitable ferrous metals include iron, low carbon steel and other iron-rich alloys but ferrous alloys containing manganese or other elements which dissolve in aqueous caustic and contaminate it are preferably avoided. Particularly effective non-ferrous, amalgam-resistant metals suitable for the overlying metal are nickel and cobalt of Group VIII of the Periodic System and chromium, molybdnum and tungsten of Group VI of the Periodic System. Also suitable are beryllium, germanium, antimony, francium, tantalum, titanium, uranium, vanadium and zirconium which are included among those metals which form satisfactory amalgam-resistant coatings.

Suitable flame-spraying devices for use according to the present invention include metallizing spray guns of the wire type, powder type or plasma type but particularly satisfactory results have been obtained using spray guns of the wire type.

v Suitably from to 90 percent of the surfaceof the graphite balls is covered by the metallizing but from 20 to '80 percent is preferred since the activity depends on exposure of both the metal surfaces and the graphite surace, to the amalgam and the hydroxylic liquid,

It is usually necessary to wash the graphite balls With 'water to remove any soluble material on the graphite which interferes with forming a tightly adherent metal-togr'aphite bond. The washed graphite balls are dried and then flame-sprayed.

f-Experimentally, a 2" deep carbon steel pan, x inches, was filled with 15 mm. graphite balls covered with A" heavy wire mesh. The flame-spraying torch was held 6" above the screen and played across the contents for 15 seconds. The halls roll under the blast from the spray gun and each is partially coated. On a larger scale, the graphite balls are suitable passed through a channel under the gun. Continuously, the balls pass under a first gun fed with iron wire and then under a second gun feed with nickel wire. When a thicker coating of the metals is desired, without completely covering the graphite, the 'balls are suitably held under a wire screen or by other means which prevent rolling and both metals are sprayed on one-half of each ball to the desired thickness, leaving the graphite uncovered on one side.

When the washed and dried graphite balls are flamesprayed with the non-ferrous metal directly, the bond is poor and in use the metal flakes off and the activity ing to the present invention is that the inventory of mercury in the decomposed is greatly reduced. During the operation of an E-510 decomposer, 36" in diameter With a bed of crushed graphite 2 feet deep, packed with A" to /2" crushed graphite, the average mercury inventory within the packing is about 500 pounds. As Example II shows, when the packing is replaced with 15 mm. graphite balls, the inventory of mercury is reduced by 324 pounds or about 65 percent.

A still further advantage of the packing of the present invention is that the vastly greater activity of the novel packing permits the use of much smaller decomposers with the larger cells, still obtaining efficient denuding. Thus an E-510 decomposer packed with the coated graphite balls of the present invention is adequate for use with the E-8l2 cell having twice the capacity of the iE-5 10 cell. The saving in mercury inventory in the decomposer over the decomposer filled with crushed graphite and usually used with the E-812 cell is about 600 pounds or 80 percent. In a plant having 100 cells, the dollar value of the investment in mercury inventory is reduced by use of the present invention by about 400,000.

The E-510 cell, earlier called E-l 1, is fully described in the art; see, for example, Electrochemical Technology, Vol. 1, pp. 71-76 (1963). The E-8l2 cell is similar, except that the anode area and capacity of the cell is approximately doubled. In addition, the cross-sectional area of the decomposer is approximately doubled.

EXAMPLE I Preliminary tests leading to the present invention are shown in Table I. Tests No. 1, 2 and 3 consisted of placing the graphite balls in a vessel attached to a gas burette and containing 0.31 percent sodium amalgam covered with about 2" of percent aqueous caustic soda at 90-95 C. and measuring the evolved hydrogen at atmospheric pressure and temperature. The metallized graphite balls of Test No. 3 had over 700 times the activity of the unmetallized balls, although the manganese in the caustic was undesirable.

TABLE I Metallizmg graphite balls (15 mm. diam.)

Time L Test number Condition of balls min: H2, cc hoii fs Remarks Previously used, Wet, n0 metal 7 1. 2 Short Wet as In 1, non-sprayed 7 8. 0 Short Iron flaked off.

Washed, dr ed, 1ron-sprayed 2 75. 0 Mn in caustic. 4 Washed, dried, nickel-sprayed 28 Nickel flaked ofi.

quickly deteriorates. However, the iron-graphite bond is E A [I tightly adherent to the washed and dried graphite and the metal-to-metal bond is also strong. The resulting dual coated graphite balls have long life, high conductivity and excellent decomposing activity in denuding amalgams resulting in high temperatures, high rates of flow of amalgam and of decomposing liquid without flaking of the ,metal coating, attrition or loss of activity. The coated graphite balls of this invention are appropriately used, if

desired, in conjunction with other known activating means, for example, addition of minor amounts of sodium molybdate to the water charged to the decomposition.

It is a particular advantage of the present invention that the residence times of the amalgam in the decomposer is materially reduced and the throughput of amalgam and decomposing liquid is materially increased. A commercial E-510 cell had been operated using a decomposer having a diameter of 36" and filled to a depth of 2 feet with A1" to /2" crushed graphite. When the graphite was replaced by 15 mm. graphite balls coated according to this invention, the residence time of the amalgam in the decomposer was reduced from 28 to 36 seconds to less than 4 seconds, greatly increasing the throughput but still efiec tively removing the sodium. Concentrations up to 0.7 per- The crushed graphite packing in a decomposer attached to an E-5l0 cell was replaced with washed, dried, ironsprayed, nickel-sprayed 15 mm. graphite balls. The decomposer attached to a commercial E-5l0 cell has a bed of graphite about 24" deep and 36" in diameter. The saving in mecury inventory due to this replacement of packing in the decomposer was 324 pounds which, at $6.90 per pound, has a value of $2232. After two months (1440 hours), the packing was continuously producing 62 percent aqueous caustic soda at 134,000 amperes, denuding the amalgam from 0.44 percent to about 0.001 percen sodium.

EXAMPLE III Tests using the procedure of Example II showed that amalgams containing 0.45 percent sodium were satisfactorily denuded at mercury flow rates up to 3500 pounds per minute which is more than the flow rate in a commercial E-812 cell of about 2200 pounds per minute. The E-8l2 cell is about 6' x 48 and operates at about 250,000 amperes. It has a decomposer with a bed of crushed graphite about 42" in diameter and 24" deep.

An E-SlO decomposer was packed with dual metallized 15 mm. graphite balls as in Example II and it replaced the larger decomposer filled with crushed graphite as normally used with the E-812 cell. The saving in mercury inventory in the decomposer was 600 pounds per E812 cell, having a value of $4464.

What is claimed is:

1. Graphite balls having over a portion of their surface a tightly adherent dual coating of underlying ferrous metal and an overlying coating of non-ferrous, amalgamresistant metal.

2. Graphite balls as claimed in claim 1 in which said dual coating covers from 10 to 90 percent of said surface.

3. Graphite balls as claimed in claim 1 having a diameter of from 5 to 50 millimeters.

4. Graphite balls as claimed in claim 1 in which said overlying coating is selected from the group consisting of cobalt, nickel, chromium, molybdenum and tungsten.

5. Graphite balls as claimed in claim 1 in which said ferrous metal is iron.

6. Graphite balls as claimed in claim 5 in which said overlying coating is nickel.

7. Method of forming coated graphite balls as claimed in claim 1 in which said balls are washed with Water, dried and flame-sprayed first with ferrous metal and then with said non-ferrous metal.

8. Method as claimed in claim 7 in which said ferrous metal is iron and said dual coating covers from 10 to 90 percent of said surface.

9. Method as claimed in claim 7 in which said nonferrous metal is nickel.

10. Method of decomposing an alkali metal amalgam to form an alkali metal hydroxide mercury and hydrogen by contacting said amalgam with hydroxylic liquid and graphite balls as claimed in claim 1.

11. Method as claimed in claim 10 in which said alkali metal amalgam is sodium amalgam containing from 0.01 to 0.7 percent by weight of sodium.

12. Method as claimed in claim 11 in which the sodium content of the effluent mercury is reduced to less than 0.1 percent.

13. Method as claimed in claim 10 in which said hydroxylic liquid is water.

14. Method as claimed in claim 13 in which the efliuent aqueous alkali metal hydroxide has a concentration above percent by weight.

15. Method as claimed in claim 14 in which said efii-uent aqueous alkali metal hydroxide is caustic soda.

US. Cl. X.R.

mg UNlTEl) STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,595,614 D d July 27, 1971 Inventofls) James M. Ford et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 1, "1920" should read --1902--; line 59, "500" should read --50--.

Column 3, line 3, after the word "beryllium" insert --gallium--;

line 28, "suitable" should read --suitably--; line 30, "feed" should read --fed--; line 73, "thus" should read --than--.

Column 4, line 22, 400,00" should read -$400,000--; line 32,

after the word "ing" insert --one of--.

Column 5, line 29, after the word "hydroxide" insert a comma Signed and sealed this 28th day of November 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTI'SCHALK Attesting Officer Commissioner of Patents 

